1. Field of the Invention
The present invention relates to a hydrostatic transmission provided with a closed circuit for connecting a hydraulic pump and a hydraulic motor. Relief valves and check valves are inserted into one side circuit and another side circuit (a low pressure side oil passage and a high pressure side oil passage) of the closed circuit.
2. Related Art
Conventionally, according to the Japanese Patent Publication Gazette No. Sho 61-27624, in a hydrostatic transmission provided with a supply circuit for supplying oil to a closed circuit comprising one side circuit and another side circuit for connecting a hydraulic pump and a hydraulic motor, each side circuit is integrally provided with three members: (1) a direct-acting relief valve for regulating maximum pressure in the closed circuit; (2) a check valve for allowing oil supply only to the low pressure side of the closed circuit; and (3) a by-pass valve for short-circuiting the closed circuit to put the hydraulic pump in the no-load state. In brief, a valve body of the check valve itself is disposed at a poppet portion formed on an outer periphery of a relief valve casing containing therein the relief valve. The by-pass valve is constructed so that when a plunger is pushed, the poppet portion of the relief valve casing slides in the direction of lifting from the seat surface of the valve casing. The relief valve casing is slidably provided with the plunger and screwably mounted to a center section provided with a closed circuit.
The above-mentioned conventional hydrostatic transmission, however, is difficult to adjust a set pressure of the relief valve. In other words, to adjust the set pressure of the relief valve, it is necessary to outwardly extract the relief valve casing. However, the relief valve casing, contained in the valve casing, cannot solely be extracted therefrom because the poppet portion exists at the outer periphery of the relief valve casing, whereby the valve casing must first be removed from the center section, and the relief valve casing can then be extracted.
The relief valve is built up in the valve body of the check valve with a relief spring disposed within the relief valve casing. When the relief valve operates, relief oil passes through the relief valve casing to escape from the casing toward the low pressure side of the hydrostatic transmission. The flow of relief oil is hindered by the relief spring so as to deteriorate the pressure override property of the check valve.